The invention relates to a method and a device for detecting a leakage in a fluid system of a blood treatment apparatus having an extracorporeal blood circuit, in particular a blood treatment apparatus having a dialyzer which is divided by a semipermeable membrane into a blood chamber and a dialysis fluid chamber.
Hemodialysis apparatuses with an ultrafiltration (UF) control means are now in widespread use. The UF control means allow a defined ultrafiltration quantity to be preset. It is known that a defect can cause a UF control means to lose its ability to exactly control the ultrafiltration rate. Since an ultrafiltration rate which is drastically increased or decreased by such a defect can endanger the patient""s life, known safety standards demand that a protection system be present in order to prevent an ultrafiltration which poses a danger to the patient. Monitoring of the transmembrane pressure (TMP) is accepted as one such protection system.
In practice, the balance error upon ultrafiltration can amount to at most 500 ml during a treatment session. In the case of a treatment session which lasts 4 hours, this corresponds to a mean volume flow of only 125 ml/h.
However, the development of dialyzers having high-permeability membranes, known as high-flux dialyzers, has led to a situation where monitoring of the TMP cannot detect a dangerously high or low ultrafiltration rate with sufficient resolution, as a consequence of the limited resolution of the TMP sensor.
In order to at least ensure that the treatment begins with an intact UF control system, there are commercially available means which permit a manual or automatic check of the integrity of the control system prior to treatment. This check is done by carrying out a pressure-holding test in the dialysis part of the equipment.
German Patent Application No. 42 39 937 A1 describes a method and a device for detecting a leakage in the fluid system of a hemodialysis apparatus on the basis of a pressure-holding test with which the functionality of the control system for the ultrafiltration rate can also be checked during the dialysis treatment. To carry out the pressure-holding test, the dialyzer is separated from the dialysis fluid part of the hemodialysis apparatus at periodic intervals during the dialysis, and in each case for a short time interval, and the pressure course in the dialysis fluid outside the separated dialyzer is recorded for any deviation from the stable state in the sense of a pressure-holding test. This method has proven useful in practice but has the disadvantage that the dialysis treatment has to be interrupted at regular intervals for the pressure-holding test. A further disadvantage is that a leak cannot be quantitatively recorded.
It is an object of the invention to make available a method by which it is possible in principle to detect leakages in the fluid system of a blood treatment apparatus without interrupting the blood treatment. It is also an object of the invention to make available a device which permits detection of leakages without interrupting the treatment.
The method according to the invention is based on a continuous measurement of the pressure in the fluid system of the blood treatment apparatus during a predetermined period of the duration of treatment, where this period can be a part of the duration of treatment or even the entire duration of treatment.
It has been found that above all the change in the static pressure in the fluid system of the blood treatment apparatus provides an indication of the presence of a leakage. The pressure can in principle be measured at any point of the fluid system. In a hemodialysis apparatus with a dialyzer which is divided by a semipermeable membrane into a blood chamber and a dialysis fluid chamber, it is possible to measure the pressure in the dialysis fluid line upstream and/or downstream of the dialysis fluid chamber of the dialyzer. It is also possible to carry out measurements in the blood line upstream and/or downstream of the blood chamber. The crucial factor is that the change in the pressure is continuously recorded during the blood treatment.
From the change in the pressure, the leak rates are calculated during the blood treatment at predetermined time intervals of the predetermined period of the duration of treatment. The leakage volume in the predetermined period of the duration of treatment is then calculated from the leak rates. The calculation of the leakage volume is thus effected by means of integration of the continuously calculated leak rates. In this way, the overall leakage amount is quantitatively available at all times. The leakage volume is then compared with a limit value. If the limit value is exceeded, this points to a possible malfunction.
To calculate the change in the pressure during the predetermined period, a pressure value is preferably continuously calculated in succeeding cycles and compared with a pressure reference value. If a leak suddenly occurs, the pressure signals change very quickly in a typical range of a few seconds to one minute. Leakages are reliably detected if the delay time xcex94t between the pressure value and the pressure reference value is likewise a few minutes. The pressure reference value should be the pressure value which was measured in a preceding cycle. In this way, slow pressure changes resulting from parameter drifts can be eliminated. In the case of insidious leaks, by contrast, it is possible that an initially leaktight system has a small leak rate which steadily rises over the course of time. The decisive factor here is not the gradient, but the duration of the change. Insidious leaks can be reliably detected if the pressure reference value is the pressure at the start of the predetermined period, i.e., the pressure reference value is constant.
The pressure value to be determined can be the instantaneously measured pressure in the fluid system. It is advantageous to calculate the pressure value by statistically evaluating a multiplicity of measurements in a predetermined time interval. The pressure value is preferably a mean value of the pressure values measured in an averaging interval. The averaging interval should here have a sufficient length in order to obtain characteristic measurement variables.
The measured pressure signal is generally superposed by various interference signals which are attributable in particular to the operation of the dialysis fluid and blood pump, the operation of the ultrafiltration pump, and the change-over of the balance chambers. A wide variety of methods can be used to eliminate these interference variables. As the interference variables have a characteristic signal course, they can be detected and shut out. The important factor is that the pressure change is calculated using a pressure signal which is as free as possible from interference variables.
If the leakage volume exceeds the predetermined limit value, an alarm can be given which points to a possible malfunction. However, it is also possible that a malfunction will only be assumed to exist when loss of leaktightness in the fluid system is established on the basis of the known pressure-holding test. For this purpose, when the predetermined limit value is exceeded, the blood treatment is interrupted and the pressure-holding test is carried out with the appropriate sensitivity. Thus, interruption of the blood treatment only takes place if there is a high probability of a leakage. If loss of leaktightness is detected by means of the pressure-holding test, an acoustic and/or optical alarm can be given. It is also possible to intervene in the blood treatment, in which case the blood treatment is preferably interrupted.
However, if the pressure-holding test does not reveal any loss of leaktightness, monitoring of the fluid system on the basis of the continuous pressure measurement is preferably reinitiated.
The pressure value can be calculated from the pressure measured at one or more points of the fluid system. Leakages have a particularly strong influence on the mean value between the pressure in the dialysis fluid upstream and downstream of the dialyzer. In addition, the transmembrane pressure can also be used. The pressure can also be measured only on the blood side. Thus, for example, leakages can be detected with only measurement of the pressure of the blood downstream of the blood chamber, which pressure is affected by the pressure drop on the needle, for example, as a result of the blood flowing back to the patient. In the event of a leak, some of the liquid blood stream flows through the membrane to the dialysate side, as a result of which the blood flow is reduced by this amount, so that the pressure in the blood stream downstream of the blood chamber falls.
In principle it is also possible for the monitoring to be carried out using a plurality of pressure values based on different pressure measurements. In this way a plausibility check is possible. For example, a particular indication of a leakage in this case is if the transmembrane pressure rises and the pressure in the blood stream downstream of the blood chamber falls.
It has been found that the ultrafiltration rate and the leak rate are equivalent disturbances in respect to the static pressure. Therefore, the influence of a leakage on the pressure in the fluid system can be simulated prior to treatment, in terms of amount and direction, by varying the UF rate. During treatment, the measurement values can then be compared accordingly.
The device for detecting a leakage has a controlling and computing unit, means for recording the pressure measurement values, means for calculating the leak rates, means for calculating the leakage volume, and means for comparing the leakage volume with a limit value. Such means can be provided by a microprocessor with corresponding software and suitable pressure sensors. Since a microprocessor and pressure sensors are present anyway in the known blood treatment apparatuses, the technical outlay is low.